Cancer stem cells have recently been identified in several different malignancies. An example is our finding that the hallmark of multiple myeloma (MM), the neoplastic plasma cells (PC), have limited replicative potential;rather, the MM PC actually arise from self-renewing cancer stem cells that resemble memory B cells. Yet, there have been limited data on the clinical relevance of cancer stem cells. We found that the novel anti-MM agents bortezomib (velcade) and lenalidomide (revlimid) inhibited MM PC but had little activity against MM stem cells in vitro. Conversely, rituximab and alemtuzumab eliminated MM stem cells in vitro, but had no activity against MM PC that lack the relevant target antigens (CD20 and CD52, respectively). In addition, we and others have shown that imatinib has little to no activity against chronic myeloid leukemia (CML) stem cells, despite having potent activity against committed CML progenitors from the same patients. Thus, even when the initiating oncogenic event is targeted, as with imatinib and BCR-ABL, inherent properties of stem cells may make the target inaccessible or unnecessary for cell survival. Accordingly, CML patients with the best responses to imatinib (PCR negativity for BCR-ABL) often, if not invariably, relapse when the drug is discontinued, and many have evidence of progression despite remaining on the drug. Many current therapies for cancer primarily target differentiated cancer cells that constitute the bulk of the tumor mass, rather than the rare cancer stem cells responsible for tumor maintenance. Such therapies may produce dramatic responses, but are unlikely to result in long-term remissions if the cancer stem cells responsible for maintaining the disease are also not targeted. Just as importantly, therapy directed against targets uniquely expressed by cancer stem cells might be prematurely abandoned if clinical activity is judged solely by standard response criteria that reflect the effects of treatment on the bulk of the cancer. The overall objective of this project is to explore approaches in the laboratory that target cancer stem cells in MM and myeloid malignancies, and translate promising treatments to the clinic. Thus, both laboratory studies and novel clinical trials are proposed in this project. Successful translation will require the development of novel methodologies for studying these rare cells both in the laboratory and clinically.